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Nelson in-ground sprinkler replacement

Hi, I am hoping someone can explain what is happening. A neighbor ran over one of my sprinkler heads recently with his lawnmower (not his fault if it was sticking up), but when I dug it up and tried to replace it, neither a 1/2" dia. or 3/4" dia. thread seems to work. The 1/2" thread was obviously way too small, and the 3/4" (Rain Bird) looks right.... looks the same as the Nelson head that I dug up.. but it will NOT thread onto the hose buried below.

I am new at this, so maybe I am missing some info. Is the Nelson proprietary? Will nothing else thread onto it?
I was told buy the repairman that replaced one for me last year that they are very interchangeable.

The existing (broken) Nelson head screws on and off just fine. The Rain Bird replacement won't. I tried changing the angle, pushing harder so it would catch, etc.

Maybe remove the fitting that is in the ground and replace with one that fits the Rainbird.

Thanks.
I was hoping it wouldn't take that much work. That maybe someone would just say "oh, you can't put a Rain Bird on a Nelson fitting", so I could just go find a Nelson instead... but at closer look I think the threads are deformed or something. Even tho the Nelson head goes on fine.

Nelson sprinker heads

I know it's an old thread, but I just dealt with the exact same issue, and wish I had some of this information in advance. Those brass Nelson heads are attached to a 1/2" male thread adapter onto which you can screw a plastic head. The bottom of that adapter has the odd female thread size which you mention, and which can't be found at stores. That female thread screws onto a brass threaded riser, soldered into a copper T. You need to remove the adapter from the old sprinkler head, put a new head on top, and thread it back onto that riser.
So that adapter is like GOLD when you are trying to repair these! If you can reuse it, it's pretty easy to pop a new head on.

The metal threads on that adapter will however shred plastic sprinkler bodies easily. So you might want to take a metal pipe fitting and use it to clean up the adapter threads, so you cane hopefullt get a plastic body on without cross-threading. Or, first put a new piece of metal on that old brass adapter, then the plastic head. If the adapter is underground, and you try to thread plastic onto it, you will probably destroy the threads. Old sprinkler heads are great tools for cleaning those threads up so that you can actually thread plastic on.

If that brass riser breaks out of the copper pipe, you have to dig and either solder it back in (very difficult for me), or cut the pipe and use Shark Bite copper fittings to patch in a new tee.

Also, that old adapter can travel up and down that riser, and you can easily adjust the sprinkler hight. There is about 2" of travel on my system. So you don't have to tighten the brass adapter on the brass riser(!), you can leave at wherever height you want. It's really a clever system actually, if you need to raise or lower heads, you just spin the sprinkler head. Just be careful if it's really hard to spin the head, because you will break the solder joint of that the brass riser if you use too much force. Learned that the hard way...probably should have tried to spray oil on the joint, instead I got a big pair of channel locks, broke that solder joint, and created a lot more work for myself!

RE: Nelson sprinker heads

I know it's an old thread, but I just dealt with the exact same issue, and wish I had some of this information in advance. Those brass Nelson heads are attached to a 1/2" male thread adapter onto which you can screw a plastic head. The bottom of that adapter has the odd female thread size which you mention, and which can't be found at stores. That female thread screws onto a brass threaded riser, soldered into a copper T. You need to remove the adapter from the old sprinkler head, put a new head on top, and thread it back onto that riser.
So that adapter is like GOLD when you are trying to repair these! If you can reuse it, it's pretty easy to pop a new head on.

The metal threads on that adapter will however shred plastic sprinkler bodies easily. So you might want to take a metal pipe fitting and use it to clean up the adapter threads, so you cane hopefullt get a plastic body on without cross-threading. Or, first put a new piece of metal on that old brass adapter, then the plastic head. If the adapter is underground, and you try to thread plastic onto it, you will probably destroy the threads. Old sprinkler heads are great tools for cleaning those threads up so that you can actually thread plastic on.

If that brass riser breaks out of the copper pipe, you have to dig and either solder it back in (very difficult for me), or cut the pipe and use Shark Bite copper fittings to patch in a new tee.

Also, that old adapter can travel up and down that riser, and you can easily adjust the sprinkler hight. There is about 2" of travel on my system. So you don't have to tighten the brass adapter on the brass riser(!), you can leave at wherever height you want. It's really a clever system actually, if you need to raise or lower heads, you just spin the sprinkler head. Just be careful if it's really hard to spin the head, because you will break the solder joint of that the brass riser if you use too much force. Learned that the hard way...probably should have tried to spray oil on the joint, instead I got a big pair of channel locks, broke that solder joint, and created a lot more work for myself!

Why not take some photos of the heads you're describing and upload them? It would help expand the knowledge base, especially as some products carried different names in different parts of the country.